Bimetallic snap disc or the like

ABSTRACT

A snap disc is disclosed having peripherally spaced radially extending scallops which stiffen the disc independently of the chord height of the disc. Because of the stiffening scallops it is not necessary to utilize a large chord height when manufacturing a disc for high force or temperature operation. The disc, when formed of bimetal, can operate at high or low temperatures with a narrow temperature differential of operation. Tools for forming the disc with scallops are also disclosed.

United States Patent 1 [111 3,739,643 Gerich June 19, 1973 BIMETALLICSNAP DISC OR THE LIKE FQREIGN PATENTS OR APPLICATIONS lnventor: Anton J.Gerich, Mansfield, Ohio Therm-O-Dise Incorporated, Mansfield, OhioFiled: Dec. 6, 1971 Appl. No.: 204,882

Related US. Application Data Continuation of Ser. No. 859,853, Sept. 22,1969, abandoned.

Assignee:

References Cited UNITED STATES PATENTS 11/1959 Cordero 92/104 12/1954Gates 92/104 3/1930 Germany 73/3783 7/1946 Great Britain 92/104 PrimaryExaminerDonald O. Woodiel Assistant Examiner-Daniel M. YasichAttorney-Harold F. McNenny, Donald W. Farrington, John F. Peame et a1.

[57] ABSTRACT A snap disc is disclosed having peripherally spacedradially extending scallops which stiffen the disc independently of thechord height of the disc. Because of the stiffening scallops it is notnecessary to utilize a large chord height when manufacturing a disc forhigh force or temperature operation. The disc, when formed of bimetal,can operate at high or low temperatures with a narrow temperaturedifferential of operation. Tools for forming the disc with scallops arealso disclosed.

12 Claims, 9 Drawing Figures PATENIEU 3.739.643

sum 1 at a INVENTOR. Aura/v .l. 6&59/(7/ 1 BIMETALLIC SNAP DISC OR THELIKE This is a continuation of my copending application, Ser. No.859,853, filed Sept. 22, 1969 now abandoned.

BACKGROUND OF THE INVENTION This invention relates generally to snapdiscs and more particularly to a novel and improved snap disc and to anovel and improved method and apparatus for manufacturing such discs.

PRIOR ART In the past, snap discs have been formed with radiallyextending corrugations extending from a location adjacent to theperiphery of the disc to an opening formed in the center of the disc. Anexample of such a disc is illustrated in the U.S. Letters Pat. toSpencer, No. 1,895,591. Because of the corrugated structure, such discsprovided a substantial amount of disc material and a substantial amountof movement between the two positions of stability. Other patentsillustrating similar disc-type structures include the U.S. Letters Pat.Nos. 1,895,590; 1,972,172; 1,983,823; 2,001,553; and 2,072,847.

More recently, snap discs, whether bimetallic or monometallic, have beenformed by stretching the central part of a flat disc of metal to form adished disc having a shape generally similar to a portion of a sphere.Such discs are generally formed by a bumping process wherein the disc isplaced in a female die which supports the disc at its periphery and apunch having a spherical end is pressed against the center of the discto stretch the metal and form a dish shape. Examples of such discs areillustrated in the U.S. Letters Pat. Nos. 2,717,936; and 2,954,447.Generally, the snap movement of such non-corrugated discs is not asgreat as the snap movement of the prior corrugated type discs. However,such disc can usually be made with higher degrees of operating accuracy.

Snap discs are often formed of a monometallic material. Such snap discsare often used in fluid operated switching devices and other mechanismsrequiring a snap action. When a bimetallic material is used to form adisc, it is temperature responsive and the lateral force for causing thesnapping of the disc is provided, at least in part, by the differentialexpansion and contraction of a bimetallic material. Bimetallic snapdiscs are often used in thermostats to provide both the temperatureresponse and the snap action. The present invention is particularlyapplicable for bimetallic snap discs. However, in certain of its broaderaspects, this invention is also applicable to monometallic snap discs.

A bimetallic snap disc has two positions of stability. When thetemperature of the disc is below one predetermined temperature,determined by the manufacture of the disc, it is in one of the positionsof stability. When the temperature of the disc is raised to a secondpredetermined temperature, the disc snaps through to a second positionof stability and remains in such second position of stability so long asthe temperature of the disc remains at or above the second predeterminedtemperature. If the temperature of the disc is then lowered to the firstpredetermined temperature, it snaps back through to its first positionof stability.

The difference in temperature between the two snap temperatures is thedifferential temperature of the disc. Generally speaking, it isdifficult to manufacture simple dish shaped bimetallic snap discs foroperation at either high or low temperatures without producing a discwhich has a relatively wide differential temperature. This is becausethe chord height for a given disc material must be increased in atypical disc to provide sufficient strength or rigidity to resist thehigh thermally induced force created by either high or low temperatures.However, the chord height also tends to determine the differential inlateral forces required to return the disc to its initial position andhence large chord heights cause relatively wide differentialtemperatures in the disc.

In the past various approaches have been developed in attempting toovercome this problem and to permit the manufacture of high temperatureor low temperature discs having relatively low differentialtemperatures. One approach is to use a thicker metal so that the dischas sufficient strength to resist the thermally induced force withoutrequiring the use of a large chord height. Another approach to theproblem is to use a bimetallic material which is less active (that is, amaterial having a smaller differential expansion rate between the twometals) for forming the disc so that the thermally induced forces at agiven high or low temperature is not as great.

The first approach of using a thicker metal is undesirable in manyinstances since it tends to increase the cost of the material formingthe disc and increases the likelihood of the fatigure failure when thedisc is repeatedly cycled. The second approach of using a less activebimetallic material tends to produce difficulty by making themanufacture of the disc very critical since the thermally induced forcesavailable for operating the discs are small and very slight variationsin the disc form materially alter the operating characteristics of thedisc.

Both of these approaches reduce the problem of forming high or lowtemperature low differential temperature discs to some extent. However,even when one or more of these approaches are utilized, it has beenfound that it is almost impossible to commercially manufacturebimetallic snap discs for operation at high or low temperatures with lowdifferential temperatures of operation. For example, in the past, evenwhen utilizing one or more of the approaches described above, abimetallic disc which snaps at a temperature in the order of 350F. willhave a differential temperature of operation in the order of about F. Soit will snap back at a temperature in the order of 230F.

SUMMARY OF INVENTION tion at a temperature of 348F. and back to itsinitial.

position when the temperature dropped to 340F.

The disc in accordance with this invention is shaped so that thestifiness is obtained without excessive chord height. This stiffness, inthe illustrated embodiments, is obtained by forming shallow scallops inthe disc which extend from the edge at peripherally spaced locationstoward the center of the disc. These scallops provide disc stiffnesswithout excessive chord height. Therefore, a disc in accordance withthis invention has a relatively low chord height and consequently, arelatively low differential temperature, even though it providessufficient rigidity or stiffness to resist relatively high thermallyinduced forces. A monometallic disc in accordance with this invention isalso desirable since such disc can withstand a relatively large lateralforce before snapping without requiring a large differential in thelateral force before it snaps back to its initial position.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a snapdisc in accordance with the first embodiment of this invention;

FIG. 2 is a cross section taken generally along 2-2 of FIG. 1;

FIG. 3 is a side elevation partially in section illustrating one pair ofdies which may be utilized to form a disc of the type illustrated inFIGS. 1 and 2;

FIG. 4 is a perspective view of the dies illustrated in FIG. 3;

FIG. 5 is a perspective view of a snap disc in accordance with a secondembodiment of this invention wherein the entire periphery is formed withscallops;

FIG. 6 is a cross section taken generally along 6-6 of FIG. 5;

FIG. 7 is a perspective view of a die set which may be used tomanufacture a disc of the type illustrated in FIGS. 5 and 6; I

FIG. 8 is a side elevation in section illustrating a typical prior artsnap disc with one position of stability illustrated in full line andthe other position of stability illustrated in phantom; and

FIG. 9 is a force displacement curve illustrating the operation of atypical prior art disc and an improved disc in accordance with thisinvention.

Referring first to FIG. 8, a typical prior art snap disc whetherbimetallic or monometallic, has a shape substantially as illustrated inFIG. 8 in full line. Such disc is circular and, in some instances, isprovided with a small centrally located aperture therethrough. Discs ofthis type are generally formed by bumping a flat circular disc, tostretch the material of the disc and form the concave structureillustrated in full line in FIG. 8.

Generally, when the disc is formed of bimetal and is intended to operateat a temperature above ambient temperature, the disc is bumped on thehigh expansion side 11. The depth of penetration of the punch in thebumping operation tends to determine the upper temperature of operationof the disc.

The disc 10 is illustrated in full line in one of its positions ofstability. Assuming this disc is bimetal and is intended for operationat a temperature above ambient temperature, the high expansion side islocated on the surface 11 and the low expansion side is along thesurface 12. As the temperature of the disc is raised to approach itsupper operating temperature, the disc moves with creep action from theupper dotted line position to the full line position, the position whichis considered herein to be a first position of stability. As soon as thetemperature of the disc reaches its upper predetermined temperature ofoperation, the disc moves with snap action to the downward dotted lineposition. If the temperature of the disc is raised to still a highertemperature, the disc moves with creep or slow movement towardconditions of even greater downward concavity.

As the temperature of the disc is reduced and as the temperatureapproaches its lower predetermined temperature of operation, the discmoves toward its second position of stability indicated in phantom. Asthe temperature of the disc is reduced below the second or lowerpredetermined temperature of operation, the disc snaps back to the upperdotted line position. Here again, if the temperature is decreased stillfurther, the disc moves with creep movement beyond the upper dotted lineposition. For purposes of simplification, the two positions of stabilitymay be considered to include the adjacent positions assumed by the discwith creep action. However, the chord height discussed herein is thechord height to the inner concave surface of the disc which is presentwhen the disc is in one position of stability, and is about to snap tothe other position of stability. Therefore, the chord height h is thechord height of the disc when the disc is in the first position ofstability, but is about to snap through to the second position ofstability. Similarly, the chord height h is the chord height of the discto the inner or concave surface when the disc is in the second positionof stability, but is about to snap through to its first position ofstability. The chord height h is substantially equal to the chord heighth.

To a great extent, the temperature at which the disc snaps from thefirst position to the second position is determined by the chord heighth and h. The thermally induced force F tending to cause the disc to snapto the second position of stability is a function of temperature causedby differential expansion between the high expansion side 11 of the discand the low expansion side I 2 thereof. At higher temperatures, theforce F is greater and the chord height must be greater to resist theforce and prevent premature snap action. After the disc is snappedthrough to the second position of stability, the disc has a chord heightof h. As the temperature of the disc drops, the force F tending tomaintain the disc in the second position of stability, decreases. If thechord height h is small, the disc tends to snap back to the firstposition of stability even though a downward force F still exists. Insuch a disc, a relatively low differential temperature may exist.However, when the disc is provided with a large chord height h, it tendsto have a similar large chord height h. Therefore, such a disc tends toremain in the second position of stability and may remain in such secondposition until the temperature drops to a sufficiently low value tocause the direction of thermally induced force to reverse, asillustrated by F, before the disc snaps back to its first position ofstability. Since the chord height h must normally be relatively largewhen the disc is intended to operate at high temperatures, and since thechord height h tends to approach the chord height h, high temperaturediscs usually have a relatively wide differential temperature ofoperation.

The dotted curve in FIG. 9 illustrates a typical displacement curve of aprior art snap disc of the type illustrated in FIG. 8. In this curve,the vertical direction represents force and the horizontal directionrepresents displacement. When the disc is formed of bimetal, the forceis produced thermally and the vertical direction may be considered to betemperature since the thermally induced force tending to snap the discis a function of temperature.

Assuming the disc is bimetal, as the disc temperature is raised fromnormal room temperature to its upper operating temperature it followsthe curve from zero to the point 13. The disc moves with creep movementto a displacement position A which corresponds to the point 13 andrepresents the first position of stability reached immediately beforethe disc snaps. On increasing temperature, the disc then moves with snapaction to the position 14. If the temperature of the disc is raisedstill further, it moves up along the curve past the position 14.However, if the disc temperature is then reduced, the disc moves withcreep action to the second position of stability 15 represented by thedisplacement position B. On decreasing temperature the disc then snapsto the position 16.

The difference in temperature between the temperature at the point 13and the temperature at the point 15 represents the differentialtemperature or the AT of the disc. Generally speaking, as the chordheight is increased, the AT or difference in temperature of operationbetween the points 13 and 15 increases drastically. The solid line curvein FIG. 9 illustrates the curve of operation for a typical disc inaccordance with the present invention. This curve will be discussed indetail below.

Referring to FIGS. 1 through 4, one embodiment of this invention is adisc formed with tapered scallops 21 extending from the edge of the disc22 toward the center of the disc 23. In the disc 20 there are eightscallops 21 which are symmetrically located around the periphery of thedisc. These scallops 21 extend inwardly toward the center but terminateat points 24 spaced from the center 23 of the disc. The scallops 21 inthe disc 20 are wedge shaped and tapered from a maximum width w at theedge 22 to a substantially zero width at 24. Similarly, the height ofthe scallops 21 is at a maximum height x at the edge 22 and the height xdecreases to substantially zero at the point 24. In this embodiment, thewidth w of the scallops 21 is arranged so that the scallops 21 arespaced from each other by smoothly dished intermediate sections 26 ofthe disc. Since the disc 20 is provided with eight scallops 21, thewidth w is substantially less than one-eighth of the peripheral lengthof the disc.

As best illustrated in FIG. 2, the disc 20 is also dished upwardly adistance I: from a reference plane 27 so the principal chord height ofthe disc is represented by the distance h. The height h is exaggeratedfor purposes of illustration in FIG. 2. It should be noted however, thatthe height x of the scallops 21 is less than the chord height h so thateven the material along the ridges 28 is curved upwardly toward thecenter 23 of the disc. In most instances, the ridges 28 are formed bysharper bends than the roots 29 where the scallops blend into theintermediate sections 26. The ridges 28 and roots 29 should not be sharpenough to produce sufficient stress concentrations to produce fatiguefailure in the disc.

When the disc snaps from its upward or first position of stability,illustrated in full line in FIG. 2, to its lower or second position ofstability illustrated in phantom in FIG. 2, the scallops 21 providestiffness tending to resist such movement. After snapping through to itslower-or second position of stability, the disc has a chord height of h'which is substantially equal in size to the chord height h, but tends tobe slightly smaller since the scallops 21 tend to provide a continuingforce urging the disc back toward its first or of stability.

Since the disc tends to remain in the first position of stability whenlateral forces are not applied, a down ward force represented by thearrow F is required to move the disc to its second or lower position ofstability. Once it moves to the lower position of stability, it remainsin such position as the force F reduces. However, since the chord heighth is small, it tends to snap back to its first position of stabilitywhen the force thereon is still in a downward direction, but has a lowervalue represented by the force F The difference in absolute valuebetween the forces F and F determine the temperature difierential ofoperation when the snap disc is formed of bimetal or the differential offorce when the disc is formed of a monometallic material.

For example, if the disc is intended for high temperature operation, thelower side 31 of the disc is the high expansion side of the bimetal andthe upper side 32 is the low expansion side. As the temperature of thedisc increases, the value of the thermally induced force in the downwarddirection increases until a force having a value F is reached. At thistime, the disc snaps through to the lower position of stability andremains in that position so long as the force has a value greater thanthe force F. Since the thermally induced force in this instance is thefunction of temperature and since the absolute value of the force F isalmost as great as the absolute value of the force F, the disc tends tosnap back to its initial position when the disc temperature is reducedonly a relatively small amount. When a low differential temperature ofoperation is required, the chord heights h and h are arranged to besmall, and the principal structure for resisting snap action is providedby the rigidity of the scallops 21. In most instances the disc has arelatively low differential temperature when the chord heights h and hare less than about one percent of the disc diameter. (For example, achord height of 0.010 inch for a one inch disc.) If increased stiffnessis required for higher temperature operation, the height x of thescallops can be increased, the length of the scallops can be increasedso that they extend closer to the center 23, or the disc can be formedwith a greater number of scallops. In this way, the stiffness of thedisc can be increased without increasing the chord height. Therefore, itis possible to manufacture discs for a higher temperature of operationwithout increasing the chord height and without creating a disc having alarge temperature differential in operation. In some instances the disccan be formed so that some of the scallops extend to the center andactually join opposite scallops.

When the disc is intended for low temperature operation the disc isformed so that the high expansion side is along the surface 32 and thelow expansion side is along the surface 31. However, the function of thedisc is similar to the function described above. When the disc is notintended to be temperature responsive and is formed of monometallicmaterial, the forces F and F can be applied to the disc in any desiredmanner, for example, by fluid pressure or by a linkage. However, thesnap action between the two positions will occur with relatively smallchanges in the force level so the disc may be considered to have a lowdifferential in operation.

FIGS. 3 and 4 illustrate one form of tooling which may be used to formthe disc illustrated in FIGS. 1 and upper position 2. This toolingincludes a punch 36 and a die 37. The operating end face of the punch 36is formed with eight ribs 38 which extend from the center 39 to the edge41 of the punch. The ribs 38 are tapered back from the center 39 by anangle a with respect to a plane perpendicular to the punch axis. Betweenthe ribs 38, the end face of the punch 36 is formed with relief sections42.

The die 37 is provided with an end face formed with eight radiallyextending wedges 43 which extend inwardly from the periphery 44 of thedie and terminate at ends 46 spaced from the central portion of the die.The wedges 43 are spaced from each other by grooves 47 formed in the endface ,of the die. The operating faces 48 of the wedges are formed at anangle b with respect to a plane perpendicular to the die axis and theangle b is slightly larger than the angle a.

When a flat disc is positioned in between the punch 36 and the die 37and the punch is moved toward the die, the disc is first engaged by thepunch at its center 39 and by the operating surfaces 48 at theperiphery. As the punch penetrates into the disc, the disc material isstretched until it becomes generally dish shaped with an angleapproaching the angle a. At this point, however, the disc still onlyengages the operating surfaces 48 substantially at their periphery.Further continued movement causes the outer portions of the ribs 38 toenter the spaces 47 between the wedges 43 to form the scallops.Therefore, the formation of the scallops commences at the periphery andthe maximum scallop height x occurs at the periphery.

It has been found that a disc may be formed with a single bumping withthese tools from only one side, and that such disc can be a snap disc inthat it has two positions of stability and moves between them with snapaction. If desired, however, the disc may be subsequently bumped fromthe other side preferably with a normal spherical tool and a cooperatingdie which supports the disc at its periphery. This secondary bumpingtends to increase the operating temperature differential to the desiredlevel.

FIGS. and 6 illustrate another embodiment of the snap disc incorporatingthis invention. The disc 60 of this embodiment is formed with scallops61 which have sufficient width to join each other without intermediateflat sections. In the disc 60 there are again eight scallops whichextend from the edge of the disc toward the center 62, but terminate atlocations 66 spaced from the center. Since the scallops 61 are shaped sothat when each scallop joins with the adjacent scallop, the width of thescallop w at the edge of the disc is equal to the chord width ofone-eighth of the disc periphery. Each of the scallops includes a ridgebend 63 and a pair of root bends 64. The scallops 61 have a height xwhich is a maximum adjacent to the edge of the disc and tapers to a zeroheight at the inner ends 66 of the scallops. Here again, even the ridges63 are curved slightly since the height x is lees than the chord heighth of the disc. In FIG. 6 the height h is again exaggerated for purposesof illustration.

The angle of the root bends 64 with respect to the reference plane 67 isgreater than the angle of the ridges 63 and the central portion of thedisc inwardly of the ends 66 of of the scallops 61 is generally smoothhaving a dish shape which is approximately spherical.

When the disc 60 snaps through from the first position of stability,illustrated in full line in FIG. 6, to the second position of stability,illustrated in phantom, it

has a chord height h which is substantially equal to the chord height h.Since the side walls of the scallops of this embodiment are not assteep, the rigidity provided thereby tends to be less than theembodiment of FIGS. 1 and 2. The operation of the disc is similar,however, to the disc of the embodiment of FIGS. 1 and 2 in that a discof the embodiment of FIGS. 5 and 6, formed of bimetal, can bemanufactured for relatively high or low temperature operation with a lowdifferential temperature of operation. Similarly, a monometallic disc inaccordance with the embodiment of FIGS. 5 and 6 can be manufactured torequire a relatively large operating force with a relatively smalldifferential force in operation.

The disc illustrated in FIGS. 5 and 6 can be manufactured on a die setas illustrated in FIG. 7. The punch 71 is formed with an end face withnarrow ridges 38' similar to the punch 41 of FIG. 4. However, the die 72is formed with narrower wedge faces 73 and wider relief spaces 74. Thewedge faces 73 are quite narrow so the roots 64 of the scallops arerelatively sharp. Since the disc is not supported over a substantialarea, the intermediate sections are not formed by the die set of FIG. 7.Here again, the wedge faces 73 extend toward the center of the die, butterminate at a location spaced therefrom. Also, the angle of the diefaces 73 with respect to the plane perpendicular to the axis of the die72 is steeper than the angle of the ribs 38' of the punch 71 withrespect to the plane perpendicular to the punch axis. Therefore, thescallops have a maximum depth or height x at the edge of the disc andhave decreasing height x as they extend toward the center of the disc.The dies of both embodiments form the scallops primarily by stretchingthe metal and do not materially change the disc diameter.

Here again, a snap disc can be formed with the die set of FIG. 7 by asingle bumping operation. However, bumping with a conventional sphericalpunch in a conventional peripheral supporting die in the oppositedirection may be utilized to increase the differential temperature ofoperation to a desired value. It has been found that the force of a discin accordance with this embodiment is slightly less than the force of adisc having similar operating temperatures, but formed in accordancewith the embodiment of FIGS. 1 and 2.

Referring again to FIG. 9, the solid line curve 81 represents the forceor temperature displacement curve of a typical disc incorporating thisinvention. In this disc, the disc moves with creep action from the zeropoint to the point 82 on increasing force, or temperature in the case ofa bimetallic disc. At the position of stability at 82, further increasesin temperature or force causes the disc to snap to the point 83. Ondecreasing temperature or force, the second position of stability isreached at 84 and the disc snaps back to the point 86. In such discs,the differential temperature of operation or the differential force isrepresented by AT. A comparison of the two curves will illustrate thedifferential temperature of operation is much less even though the discis a higher temperature disc than the conventional disc illustrated inthe dotted line. It appears that a disc formed with a shape inaccordance with this invention has a positive spring rate superimposedon the normal spring rate of a conventional disc of the type having acurve, as illustrated by the dotted line.

Although the two illustrated embodiments each have eight scallops, discsmay be formed with greater or lesser numbers of scallops. Generally,smaller diameter discs are formed with a smaller number of scallops thanlarger diameter discs.

Since a bimetallic snap disc in accordance with this invention can bemanufactured for high or low temperature operation with a relativelysmall differential temperature of operation without using thickbimetallic material, fatigue failures are not a problem. Also, arelatively active bimetallic material of the type used for conventionaldies can be utilized Because the disc can be made without the use ofspecial materials, inventory costs for manufacture are reduced and inmany instances, discs can be made which were difficult or impossible tocommercially manufacture with conventional prior art practices.

Although preferred embodiments of this invention are illustrated, it isto be understood that various modifications and rearrangements may beresorted to without departing from the scope of the invention disclosed.

What is claimed is:

l. A snap disc comprising a piece of metal, said metal being dished toprovide a dished portion of shallow concavity which is substantiallysymmetrical about its center, said dished portion being formed with aplurality of tapered shallow scallops spaced around the periphery ofsaid portion and converging generally radially from said peripherytoward said center to inner ends, said disc being movable with snapaction between two positions of stability in which said disc hasopposite concavity, said scallops increasing the stiffness of said discwhen compared to a similar disc without scallops and substantiallyincreasing the discs resistance to movement from one of said positionstoward the other of said positions, said scallops being substantiallywedge shaped and tapered substantially uniformly from a maximum width atsaid periphery to a minimum width at said inner ends and taperedsubstantially uniformly from a maximum height at said periphery tosubstantially zero height at said inner ends, said inner ends of saidscallops blending into and being spaced from each other by a smoothlydished central portion.

2. A snap disc as set forth in claim 1 wherein said scallops are formedby opposed substantially planar side walls joined together at the ridgebend and joined to the adjacent portions of said metal opposite saidridge bends at root bends.

3. A snap disc as set forth in claim 2 wherein said scallops are taperedsubstantially uniformly to substantially zero width at said inner ends,said side walls are substantially triangular, each root bend of eachscallop is spaced from the adjacent root bend of the adjacent scallop.

4. A snap disc as set forth in claim 2 wherein each scallop is joinedalong its edges to the adjacent scallop.

A snap disc comprising a piece of metal, said metal being dished toprovide a dished portion of shallow concavity which is substantiallysymmetrical about its center, said dished portion being formed with aplurality of tapered shallow scallops spaced around the periphery ofsaid portion and extending generally radially toward said center toinner ends, said disc being movable with snap action between twopositions of stability in which said disc has opposite concavity, saidscallops increasing the stiffness of said disc when compared to asimilar disc without scallops and substantially increasing the discsresistance to movement from one of said positions toward the other ofsaid positions, said scallops being substantially wedge shaped andtapered substantially uniformly from a maximum width at said peripheryto a minimum width at said inner ends and tapered substantiallyuniformly from a maximum height at said periphery to substantially zeroheight at said inner ends, said metal being bimetal and said snap discsnaps between said positions of stability in response to changes in thetemperature thereof.

6. A snap disc as set forth in claim 5 wherein said scallops are formedby opposed substantially planar side walls joined together at the ridgebend and joined to the adjacent portions of said metal opposite saidridge bends at root bends.

7. A snap disc as set forth in claim 6 wherein said scallops are taperedsubstantially uniformly to substantially zero width at said inner ends,said side walls are substantially triangular, each root bend of eachscallop is spaced from the adjacent root bend of the adjacent scallop.

8. A snap disc as set forth in claim 6 wherein each scallop is joinedalong its edges to the adjacent scallop.

9. A snap disc as set forth in claim 5 wherein said disc is imperforateand said inner ends of said scallops are spaced from each other by asmoothly dished central portion.

10. A snap disc as set forth in claim 5 wherein said inner ends of saidscallops are spaced from each other by a smoothly dished portion of saiddisc.

11. A snap disc comprising a generally circular piece of bimetal dishedto a shallow concavity which is substantially symmetrical about thecenter thereof, said bimetal being formed with a plurality of taperedshallow scallops spaced around the periphery thereof extending generallyradially toward the center of said disc to inner ends, said disc beingmovable with snap action between two positions of stability in whichsaid disc has opposite concavity, said scallops increasing the stiffnessof said disc when compared to a similar disc without scallops andsubstantially increasing the discs resistance to movement from one ofsaid positions toward the other of said positions, said scallopsproviding a maximum stiffness at said periphery and a stiffness whichdecreases substantially uniformly to said inner ends, said scallopsbeing substantially wedge shaped and tapered substantially uniformlyfrom the maximum width of said periphery to a minimum width at saidinner ends and tapered substantially uniformly from a maximum height atsaid periphery to a substantially zero height at said inner ends.

12. A snap disc as set forth in claim 11 wherein said maximum height ofsaid scallops is substantially less

1. A snap disc comprising a piece of metal, said metal being dished toprovide a dished portion of shallow concavity which is substantiallysymmetrical about its center, said dished portion being formed with aplurality of tapered shallow scallops spaced around the periphery ofsaid portion and converging generally radially from said peripherytoward said center to inner ends, said disc being movable with snapaction between two positions of stability in which said disc hasopposite concavity, said scallops increasing the stiffness of said discwhen compared to a similar disc without scallops and substantiallyincreasing the disc''s resistance to movement from one of said positionstoward the other of said positions, said scallops being substantiallywedge shaped and tapered substantially uniformly from a maximum width atsaid periphery to a minimum width at said inner ends and taperedsubstantially uniformly from a maximum height at said periphery tosubstantially zero height at said inner ends, said inner ends of saidscallops blending into and being spaced from each other by a smoothlydished central portion.
 2. A snap disc as set forth in claim 1 whereinsaid scallops are formed by opposed substantially planar side wallsjoined together at the ridge bend and joined to the adjacent portions ofsaid metal opposite said ridge bends at root bends.
 3. A snap disc asset forth in claim 2 wherein said scallops are tapered substantiallyuniformly to substantially zero width at said inner ends, said sidewalls are substantially triangular, each root bend of each scallop isspaced from the adjacent root bend of the adjacent scallop.
 4. A snapdisc as set forth in claim 2 wherein each scallop is joined along itsedges to the adjacent scallop.
 5. A snap disc comprising a piece ofmetal, said metal being dished to provide a dished portion of shallowconcavity which is substantially symmetrical about its center, saiddished portion being formed with a plurality of tapered shallow scallopsspaced around the periphery of said portion and extending generallyradially toward said center to inner ends, said disc being movable withsnap action between two positions of stability in which said disc hasopposite concavity, said scallops increasing the stiffness of said discwhen compared to a similar disc without scallops and substantiallyincreasing the disc''s resistance to movement from one of said positionstoward the other of said positions, said scallops being substantiallywedge shaped and tapered substantially uniformly from a maximum width atsaid periphery to a minimum width at said inner ends and taperedsubstantially uniformly from a maximum height at said periphery tosubstantially zero height at said inner ends, said metal being bimetaland said snap disc snaps between said positions of stability in responseto changes in the temperature thereof.
 6. A snap disc as set forth inclaim 5 wherein said scallops are formed by opposed substantially planarside walls joined together at the ridge bend and joined to the adjacentportions of said metal opposite said ridge bends at root bends.
 7. Asnap disc as set forth in claim 6 wherein said scallops are taperedsubstantially uniformly to substantially zero width at said inner ends,said side walls are substantially triangular, each root bend of eachscallop is spaced from the adjacent root bend of the adjacent scallop.8. A snap disc as set forth in claim 6 wherein each scallop is joinedalong its edges to the adjacent scallop.
 9. A snap disc as set forth inclaim 5 wherein said disc is imperforate and said inner ends of saidscallops are spaced from each other by a smoothly dished centralportion.
 10. A snap disc as set forth in claim 5 wherein said inner endsof said scallops are spaced from each other by a smoothly dished portionof said disc.
 11. A snap disc comprising a generally circular piece ofbimetal dished to a shallow concavity which is substantially symmetricalabout the center thereof, said bimetal being formed with a plurality oftapered shallow scallops spaced around the periphery thereof extendinggenerally radially toward the center of said disc to inner ends, saiddisc being movable with snap action between two positions of stabilityin which said disc has opposite concavity, said scallops increasing thestiffness of said disc when compared to a similar disc without scallopsand substantially increasing the disc''s resistance to movement from oneof said positions toward the other of said positions, said scallopsproviding a maximum stiffness at said periphery and a stiffness whichdecreases substantially uniformly to said inner ends, said scallopsbeing substantially wedge shaped and tapered substantially uniformlyfrom the maximum width of said periphery to a minimum width at saidinner ends and tapered substantially uniformly from a maximum height atsaid periphery to a substantially zero height at said inner ends.
 12. Asnap disc as set forth in claim 11 wherein said maximum height of saidscallops is substantially less than the chord height of the dish in saiddisc.